1. Field of the Invention
The present invention is directed to a bioinstrumentation device for measuring a biological parameter of a living body, and more particularly for measuring a heart rate and a breathing rate of a human subject without giving any annoying constriction to the subject.
2. Description of the Prior Art
It has been known in the art to measure a biological parameter such as heart rate and the breathing rate from periodical minute variations or vibrations appearing in the weight of a human subject as a consequence of the heart beat and the breathing of the subject. Japanese Patent Publication No. 6-30914 proposes to use an optical fiber for sensing such minute variations based upon the finding that optical fiber will change its internal reflectivity as it is caused to deform under the influence of the weight or load variations. The change of the internal reflectivity results in corresponding change in the amount of the light transmitted through the optical fiber which can be readily analyzed to determine the heart rate and the breathing rate. However, there still remains a problem to be solved in order to achieve sufficient sensitivity or S/N ratio for accurately determining the heart rate and the breathing rate. The problem concerns that when the optical fiber is kept generally straight, no remarkable change in the reflectivity develops with a slight deformation of the optical fiber from the straight condition, thereby failing to derive a reliable indication of the minute load variations representing the biological parameters.